1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery, and more specifically, to a nonaqueous electrolyte battery using calcium as its active material.
2. Description of the Related Art
Conventionally, in researches for secondary batteries with high energy density, nonaqueous electrolyte secondary batteries which use a nonaqueous electrolyte and make lithium ions transfer between a positive electrode and a negative electrode to achieve charge/discharge have been vigorously studied.
In recent years, as batteries to be used in portable electronic/communicative equipments such as small-sized video camera, portable phone, laptop and the like, nonaqueous electrolyte batteries represented by a lithium-ion battery are expected for practical use as batteries which are small in size and weight and enable charge/discharge with a large capacity. A commonly used lithium-ion battery uses an alloy, a carbon or silicon material capable of absorbing, storing and discharging lithium ions, or the like as a negative electrode active material, layer-shaped lithium cobalt (LiCoO2), lithium transition metal complex oxides such as lithium nickel oxide (LiNiO2), spinel-shaped lithium manganese oxide (LiMn2O4) as a positive electrode active material, and a solution of electrolyte comprising lithium salts such as LiBF4 and LiPF6 dissolved in an organic solvent such as ethylene carbonate and diethyl carbonate.
Also proposed is a battery which uses carbon fluoride as a positive electrode active material, a negative electrode formed of an alkaline metal such as lithium and sodium, and a nonaqueous electrolyte (see Japanese Patent Publication No. 48-25566).
Also proposed is a battery which uses sulfur as a positive electrode active material, a negative electrode formed of an alkaline metal such as lithium, sodium and the like, and a nonaqueous electrolyte (see Japanese Patent Publication Laid Open 2002-75446).
On the other hand, from the view point of energy density, researches are made in which alkaline earth metals such as magnesium and calcium or light metals such as aluminum are used as a negative electrode active material.
When calcium ions are used as an ion conductive medium in place of lithium ions, there arises an advantage that the number of reactive electrons is large and the cost is inexpensive.
Although batteries using such calcium ions as an active material have been suggested, any of the batteries using calcium ions as an active material that have been reported and practically used employ sulfides or oxides for a positive electrode and have a capacity of as small as 200 mAh/g or less.
Furthermore, batteries that are practically used and employ calcium ions as an active material used perchlorates such as calcium perchlorate (Ca(ClO4)2) as an electrolyte.
However, since calcium perchlorate is a salt of peroxide, and hence chemically unstable and likely to release oxygen, it is too risky to be actually used as an electrolyte. Therefore, a nonaqueous electrolyte battery using such a peroxide salt of calcium as an electrolyte could not bear practical use.